LIBRARY 

OF  THE 

UNIVERSITY  OF  CALIFORNIA. 

RECEIVED    BY    EXCHANGE 

Class 


An     Electrolytic    Method    of 
Analyzing  Zinc  Ores 


THESIS 

Presented  to  the  Faculty  of  the  Department  of  Philosophy 

of  the 

University   of  Pennsylvania 
in  Partial  Fulfilment  of  the  Requirements 

for  the  Degree  of 
Doctor  of  Philosophy, 

BY 

HARRISON  HALE 

SPRINGFIELD,  Mo. 

1908 


HODGES  &  KIRK 

PHH.ADEI.PHIA 

1908 


An     Electrolytic    Method    of 
Analyzing  Zinc  Ores 


THESIS 

Presented  to  the  Faculty  of  the  Department  of  Philosophy 

of  the 

University   of  Pennsylvania 
in  Partial  Fulfilment  of  the  Requirements 

for  the  Degree  of 
Doctor  of  Philosophy, 


HODGES  &  KIRK 

PHILADELPHIA 
1908 


This  work  was  done  at  the  suggestion  of  Dr.  Edgar  F.  Smith, 
and  the  writer  wishes  to  express  most  sincere  thanks  both  for  the 
assistance  given  in  this  investigation  and  for  unfailing  personal 
kindness. 


INTRODUCTION. 

The  application  of  electro-analysis  with  its  accuracy,  its  clean- 
liness and  its  ease  of  execution  to  the  determination  of  mineral 
constituents  has  obvious  advantages.  This  has  long  been  recog- 
nized. More  than  thirty  years  ago  Parodi  and  Mascazzini  (i) 
published  a  method  for  the  determination  of  zinc  by  adding  an 
excess  of  ammonium  acetate  to  a  solution  of  its  sulphate;  this 
method  was  recommended  for  use  in  the  analysis  of  zinc  ores. 

In  1902  Edgar  F.  Smith  (2)  after  expressing  confidence  that 
electrolytic  methods  would  prove  "wholly  satisfactory"  on  ap- 
plication to  natural  products  gives  results  of  the  determination 
of  zinc  in  a  pure  blende,  using  a  sodium  acetate  electrolyte,  the 
zinc  being  present  as  sulphate.  The  introduction  of  the  rotating 
anode  with  zinc  solutions  by  Exner  (3),  greatly  increasing  the 
advantages  of  the  electrolytic  method  has  made  an  imperative  de- 
mand for  further  work  along  this  line. 

With  these  facts  in  view  a  study  of  zinc  blende  was  undertaken 
with  the  purpose  of  obtaining  not  only  a  method  for  the  determ- 
ination of  the  metal  but  also  of  the  sulphur  content  of  the  min- 
eral. The  results  have  shown  that  both  may  be  determined  with 
ease  from  the  same  sample  and  in  a  shorter  time  than  by  any 
other  satisfactory  process. 

Attention  has  also  been  given  to  the  form  of  electrodes,  es- 
pecially the  cathode,  best  suited  for  this  determination.  Further 
work  was  then  done  to  test  the  general  applicability  of  the  method 
for  determining  the  metal  from  any  zinc  ore  with  fair  success. 
Such  a  practical  method  is  particularly  desirable  on  account  of 
the  present  unsatisfactory  condition  of  rapid  zinc  estimation. 

(i).    Ber.,  10,  1098. 

(2).    J.  Am.  Ch.  S.  24,  1073- 

(3).    J.  Am.  Ch.  S.  25,  899- 


186924 


PRELIMINARY  WORK. 

A  pure  zinc  blende  finely  ground  was  used  to  try  out  the 
method  as  follows :  A  weighed  portion  of  the  ore  was  placed  on 
about  fifteen  grams  of  sodium  hydroxide  which  had  been  pre- 
viously fused  in  a  nickel  crucible  and  allowed  to  cool.  This 
was  then  heated  to  quiet  fusion,  the  top  of  the  Bunsen  flame 
touching  the  crucible,  for  fifteen  or  twenty  minutes.  The  heat 
should  be  strong  enough  to  completely  decompose  the  ore  but 
the  fusion  should  not  boil.  On  cooling,  the  melt  was  taken  up  in 
water,  filtered,  diluted  to  125  c.  c.  and  electrolyzed  in  a  silver- 
coated  platinum  dish  with  a  current  of  N.  D.100  =  5  amperes  and 
4  volts,  using  a  flat  spiral  anode  of  platinum  wire  (about  i  mm. 
diameter),  making  approximately  600  revolutions  per  minute. 

The  glass  covers  were  washed  down  with  hot  water  five  to  ten 
minutes  before  the  current  was  interrupted.  The  liquid  was 
siphoned  off  without  breaking  the  current,  water  being  poured 
in,  The  deposit  was  further  washed  with  water  then  with  abso- 
lute alcohol  and  ether.  (See  Smith's  Electro- Analysis  4th  edi- 
tion, p.  116).  This  gave: 

Number  Zinc  blende  Zinc  found  Zinc 

gram  gram  per  cent. 

1  0.2378          0.1549       65.14 

2  0.3842          0.2505        65.20 

3  0.3790          0.2474       65.28 

4  0.2733          0.1831        67.00 

In  No.  4  an  effort  was  made  to  determine  zinc  without  filter- 
ing— simply  taking  up  the  fusion  in  the  crucible  with  water, 
decanting  into  a  dish  and  electrolyzing.  The  high  result  was  due 
to  impurities  in  the  deposit,  iron  being  found  and  silica  also 
probably  being  present. 

The  zinc  was  removed  from  the  dish  by  dilute  sulphuric  acid 
(i :  50)  which  had  almost  no  effect  on  the  silver  coating. 


GENERAL  EXPERIMENTAL  WORK. 

For  this  work  a  larger  quantity  of  another  sample  of  pure 
blende  from  Joplin  (i)  was  ground  so  as  to  pass  through  a  120 
mesh  sieve.  The  method  was  the  same  as  in  the  preliminary 
work.  However  potassium  hydroxide  (Kahlbaum's  sticks  pure 
by  alcohol)  was  used  for  the  fusion,  it  being  considered  purer. 
The  fusion  was  taken  up  in  water  and  filtered  through  a  Gooch 
crucible  fastened  by  a  stopper  in  the  top  of  a  bell  jar,  into  a 
nickel  dish.  The  work  of  Mr.  J.  S.  Goldbaum  in  this  laboratory 
had  shown  this  to  give  better  zinc  deposits  and  to  be  more  easily 
cleaned  than  the  silver-coated  platinum  dish.  The  deposition 
was  more  rapid  if  the  solution  was  warmed  before  the  current 
was  started. 

A.  Nickel  dish  cathode,  hemispherical,  5cm.  diameter.  Solu- 
tion diluted  till  the  cathode  surface  was  130140  square  cm. 
Flat  spiral  anode  of  platinum  wire  as  in  preliminary  work.  These 
results  were  obtained: 


Number    Zinc  blende 
gram 


0.4513 
0.6520 
0.7279 
0.5640 
0.4242 
0.3871 
0.5557 
0.4686 
0.6325 
0.3914 
0.4033 
0.4058 
0.4852 
0.4264 
0.4226 


9 
10 

ii 

12 
13 

14 

3 
II 

19 


Time  heated  Current  Zinc  found    Zinc 

minutes       Volts  Amperes  Time        gram          per 

cent. 
30 


So 
30 
60 


35 
4p 

30 


Average — 15  determinations     64.87 


4            5 

;       30 

0.2937 

65.08 

3-5 

« 

0.4215 

64.65 

5 

45 

0.4766 

65.48 

»>           < 

30 

0.3622 

64.22 

«           < 

« 

0.2732 

64.40 

ti           t 

50 

0.2507 

64.76 

(€                          ( 

0.3601 

64.80 

M 

40 

0.3024 

64-53 

5-5 

60 
40 

0.4108 
0.2531 

64-94 
64.66 

4-5 

50 

0.2621 

64.99 

« 

0.2658 

65.50 

• 

30 

0.3133 

64.57 

M 

50 

0.2764 

64.82 

«(                    1 

40 

0.2772 

65.59 

In  No.  14  to  remove  crystals  of  potassium  sulphate  held  up 
by  the  zinc  (due  to  addition  of  sulphuric  acid  and  more  potas- 
sium hydroxide  to  remedy  a  poor  fusion)  and  in  Nos.  17  and  19 
to  remove  silica  (due  to  imperfect  filtration),  the  zinc  deposit 
was  dissolved  in  dilute  sulphuric  acid  (i :  50),  an  excess  of  potas- 
sium hydroxide  added  and  the  solution  again  electrolyzed. 


In  these  determinations  it  was  noted  that  a  rapid  rotation 
of  the  anode,  about  600  revolutions  per  minute,  greatly  improves 
the  character  of  the  zinc  deposit,  preventing  sponginess  and 
making  it  adherent.  With  this  precaution  beautiful  deposits 
were  generally  obtained,  smooth  and  adherent.  When  the  elec- 
trolysis is  first  begun  there  is  considerable  frothing  which  grad- 
ually diminishes,  becoming  least  when  most  of  the  zinc  is  de- 
posited. 

During  these  and  subsequent  determinations  no  attempt  was 
made  to  reduce  the  time  to  a  minimum,  the  aim  being  to  first 
test  the  efficiency  of  the  process.  After  the  melt  is  in  quiet  fusion 
almost  no  attention  is  required  for  it.  Save  for  washing  down 
the  cover  glasses  the  operator  need  give  practically  no  time  to  the 
electrolysis  after  turning  on  the  current.  The  writer  is  confi- 
dent that  fifteen  minutes  of  quiet  fusion  is  sufficient  for  half  a 
gram  of  ore  and  fifteen  to  thirty  minutes  for  the  deposition  of 
the  zinc,  varying  with  the  content  of  the  ore  and  the  volume 
of  electrolyte. 

Tests. — The  residue  on  the  Gooch  crucible  was  frequently 
tested  for  zinc  with  cobalt  nitrate  in  the  oxidizing  flame  before 
the  blowpipe  (Rinmann's  Green),  and  no  zinc  was  found  in  the 
determinations  given  in  this  paper. 

The  siphonate  was  always  tested  by  acidifying  with  hydrochloric 
acid  and  adding  potassium  ferrocyanide  with  negative  results  for 
zinc  in  the  determinations  given  above.  Raising  the  level  of  the 
liquid  and  noting  whether  there  is  a  further  deposition  of  the 
metal  is  an  aid  in  determining  when  the  reaction  is  complete; 
but  this  becomes  unnecessary  when  a  number  of  determinations 
are  being  made  as  the  analyst  soon  learns  the  length  of  time 
required. 

The  zinc  deposit  was  dissolved  off  the  dish  in  dilute  sulphuric 
acid,  a  few  drops  of  nitric  acid  added  and  the  solution  boiled. 
On  cooling,  this  was  tested  for  iron  with  potassium  ferrocyanide 
or  sulphocyanide.  No  iron  was  present  with  the  zinc  deposit  in 
any  of  the  determinations  previously  given. 

When  the  fusion  was  not  sufficiently  heated  a  cloudy  purplish 
mixture  was  obtained  on  adding  water.  This  substance  was  hard 
to  filter  and  invariably  left  zinc  in  the  residue  on  the  Gooch 
crucible.  This  exceptional  behavior  is  due  to  two  causes.  First : 


at  the  lower  temperature  the  iron  was  not  completely  changed 
to  the  insoluble  ferric  oxide,  and  some  potassium  ferrate  was 
formed.  This  gives  the  color  which  is  lost  on  making  the  mix- 
ture acid.  Second:  under  the  same  conditions  the  sulphur  was 
only  partially  oxidized,  giving  some  zinc  suphide,  sulphite  and 
thiosulphate  which  remained  on  the  filter  causing  the  test  men- 
tioned. This  is  proven  t>y  the  addition  of  hydrochloric  acid  in 
excess  of  the  potassium  hydroxide,  when  hydrogen  sulphide  was 
first  given  off  and  then  sulphur  dioxide  with  a  copious  separa- 
tion of  sulphur. 

These  difficulties  are  all  avoided  on  heating  the  fusion  to  a 
higher  temperature — not  to  boiling,  but  so  that  the  bottom  of 
crucible  is  touched  by  the  top  of  the  Bunsen  flame.  This  causes 
the  formation  of  the  insoluble  ferric  oxide  and  gives  no  insoluble 
zinc  compounds  on  the  Gooch  crucible. 

An  attempt  was  then  made  to  avoid  filtration  before  the  elec- 
trolysis. Experiment  No.  4  in  the  preliminary  work  had  shown 
that  this  could  not  be  done  in  a  dish.  Consequently  a  cylindrical 
nickel-plated  platinum  gauze  was  used  as  cathode,  and  as  anode 
a  small  spiral  of  platinum  wire,  both  being  suspended  vertically 
in  a  beaker,  the  anode  rotating  at  the  rate  of  at  least  500  revolu- 
tions per  minute. 

It  was  thought  that  ferric  oxide,  silica  and  other  solid  matter 
would  drop  to  the  bottom  of  the  beaker  and  not  adhere  to  the 
cathode.  Six  years  before  Smith  (4)  adopted  a  similar  pro- 
cedure in  electrolyzing  an  ammoniacal  copper  solution  containing 
ferric  hydrate  from  chalcopyrite.  He  found  "that  if  platinum 
dishes  be  employed  as  cathodes  to  receive  the  copper,  the  latter 
has  invariably  been  contaminated  with  iron,  but  that  if  platinum 
cylinders,  dipping  into  the  ammoniacal  solution  contained  in 
beaker-glasses,  were  used  for  cathodes  the  copper  deposited  upon 
them  was  free  from  iron." 

The  fusion  was  taken  up  in  water  and  poured  directly  into 
the  beaker  and  after  warming  was  analyzed  with  these  results: 

B.  Nickel-plated  platinum  gauze  cathode — vertical — 25  wires 
per  cm.  of  0.2  mm.  diameter,  10  cm.  long,  5  cm.  wide,  giving  a 
cathode  surface  of  approximately  150  square  cm.  A  stout  plati- 
num wire  is  split  for  a  part  of  its  length  and  riveted  over  the 
center  of  the  gauze,  the  upper  end  being  used  to  make  the  elec- 
trical connections  and  bent  twice  at  right  angles  to  be  clear  of  the 
rotating  anode.  The  gauze  was  bent  into  cylindrical  form,  but 
the  cylinder  was  not  entirely  closed,  the  distance  between  the 
(4).  J.  Am.  Ch.  S.  24,  1076. 


ends  being  one  to  two  cm.  As  anode  a  piece  of  platinum  wire, 
about  i  mm.  diameter,  which  had  been  wound  into  a  spiral  about 
an  ordinary  lead  pencil — vertical. 

Number    Zinc  blende    Time  heated  Current  Zinc  found    Zinc 

gram  minutes       Volts  Amperes  Time        gram          per 

cent. 

20      0.5064      30      4.5    5    55    0.3275   64.67 
"  '™  50    0.3127   65.40 


21  0.4720 

22  0.5526 


30    0.3612   65.36 


23  0.4752  40  55  0.3066  64.52 

24  0.4298  45  0.2804  65.24 

25  0.4235  50  0.2735  64.58 

26  0.4826  5  60  0.3146  65.18 
37  0.4454  90      5.5    4.5   45  0.2900  65.11 


65.01 
Less  iron..  0.08 

Average — 8    determinations   64.93 

A  slight  trace  of  iron  was  generally  found  in  zinc  deposit ;  this 
was  estimated  colorimetrically  with  potassium  sulphocyanide  and 
standard  iron  solution  to  be  0.08%,  which  correction  is  made 
above.  The  tests  for  zinc  in  siphonate  and  residue  resulted 
negatively. 

The  smallness  of  the  amount  of  iron  in  the  zinc  deposit  seems 
to  show  that  it  is  held  mechanically,  being  carried  by  the  zinc  in 
its  deposition.  The  agitation  of  the  solution  by  the  rotating 
anode  would  be  favorable  to  this.  This  agitation  may  account 
for  the  difference  in  the  result  from  the  work  of  Smith  on  chal- 
copyrite  when  the  copper  deposits  contained  no  iron  using  sta- 
tionary vertical  electrodes. 

It  was  thought  that  a  sheet  of  metal  might  offer  less  oppor- 
tunity for  retaining  the  iron.  Accordingly  the  following  determ- 
inations were  made : 

C-i.  Nickel-plated  sheet  platinum  measuring  approximately 
4  cm.  by  10  cm.,  giving  80  square  cm.  of  cathode  surface ;  partly 
closed  cylinder — vertical.  Same  vertical  anode  as  in  B. 

Number    Zinc  blende    Time  heated  Current  Zinc  found    Zinc 

gram  minutes      Volts  Amperes  Time        gram          per 

cent. 

27  0.4296      40      5     5    70    0.2797   65.10 

28  0.4172  .  40    0.2698   64.66 

The  nickel  showed  a  tendency  to  peel  off  from  the  platinum 
so  no  more  determinations  were  made. 

One  determination  was  tried,  using  for  the  cathode  an  in- 


verted  nickel  crucible,  the  bottom  of  which  had  been  cut  out  to 
admit  the  passage  of  the  anode.  This  was  suspended  by  a  nickel 
wire. 

C-2.  This  closed  nickel  cathode — bell-shaped — was  not  satis- 
factory, the  solution  not  being  sufficiently  agitated  by  the  rota- 
tion of  the  anode.  The  deposit  was  almost  entirely  on  the  inner 
surface  of  the  cathode,  which  did  not  afford  an  area  large  enough 
to  hold  all  of  the  zinc. 

€-3.  A  sheet  nickel  cathode  9.5  cm.  by  5  cm.,  giving  a  surface 
of  95  square  cm.,  of  similar  shape  to  that  used  in  C-i — vertical; 
cathode  suspended  by  nickel  wire.  Same  vertical  anode  as  in  B. 


Number 


29 
30 

31 
32 
33 

34 


Zinc  blende 
gram 

0.4104 
0.4132 
0.4094 
0.5024 
0.3681 
0.4084 
0.4555 
0.3946 


Time  heated  Current 

minutes      Volts  Amperes  Time 


40 
50 
40 


90 


4-5 
5 


5-5 


40 

u 

45 

40 
« 

50 
1? 


inc  founc 

1  Zinc 

gram 

per 

cent. 

0.2678 

65.25 

0.2669 

64.59 

0.2652 

64.77 

0.3291 

65.50 

0.2398 

65.14 

0.2665 

65-25 

0.2964 

65.07 

0.2579 

65.35 

65.07 

Less  iron..    0.12 


Average — (C-i,  €-3)   10  determinations 64.95 

A  trace  of  iron  found  in  the  zinc  deposits,  was  estimated  and 
corrected  for  as  in  B.  There  was  no  zinc  in  siphonate  or  residue 
left  on  filtering. 

With  the  sheet  nickel  cathode  the  zinc  was  not  always  ad- 
herent, several  determinations  being  lost  on  this  account. 

It  should  also  be  noted  that  the  cathode  surface  as  stated  for 
a  sheet  metal  electrode  is  not  comparable  to  that  for  a  gauze 
cathode. 

Average  of  33  determinations  giving  equal  value  to  each 

determination  64.91% 

Average  of  3  series  of  determinations  with  different  forms 
of  electrode,  giving  equal  value  to  each  series 64.92% 

From  the  previous  experiments  it  appears  preferable  in  the 
determination  of  zinc  by  fusion  with  alkali  and  electrolysis  of 
the  solution,  to  use  a  vertical  gauze  cathode  of  nickel,  dipping  into 
the  solution  in  a  beaker.  This  conclusion  is  based  on  these  con- 
siderations : 


10 

1.  Rapidity  of  weighing.    The  gauze  being  much  lighter  than 
the  dish  and  somewhat  lighter  than  the  sheet  metal,  quickly  comes 
to  constant  weight  on  the  balance  pan,  giving  a  more  accurate  re- 
sult with  greater  speed. 

2.  Slight  interference  of  gangue  or  other  suspended  matter. 
Indeed,  unless  the  amount  is  considerable,  the  interference  is  neg- 
ligible. This  advantage  is  greatest  over  the  dish,  where  the  electro- 
lyte to  give  the  best  results  must  be  perfectly  clear.     It  is  seen 
also  that  the  gauze  has  here  a  slight  advantage  over  the  sheet 
metal. 

3.  Large   cathode   surface   in    small    amount   of    electrolyte. 
This  advantage  is  not  only  in  the  greater  surface  of  the  cathode, 
but  the  easy  flow  of  the  solution  through  the  gauze  gives  a  more 
even  deposit  over  its  entire  surface  and  makes  a  given  surface 
of  correspondingly  greater  efficiency  than  is  possible  with  a  sheet 
of  metal  of  cylindrical  form,  even  when  the  cylinder  is  not  com- 
pletely closed.    If  the  cylinder  is  entirely  closed  the  outer  surface 
plays  a  still  smaller  part  in  receiving  the  deposit. 

4.  Rapidity  of  deposition  and  adherent  character  of  deposit. 
The  rapidity  of  the  deposition  is  a  result  of  the  increased  cathode 
surface   allowing   the   use   of   a   more   concentrated   electrolyte, 
while  the  gauze  offers  but  little  opportunity  for  the  zinc  to  peel 
off,  which  danger  is  to  be  guarded  against  with  the  other  forms 
of  cathode. 

At  the  end  of  the  electrolysis  the  vertical  cathode,  either  gauze 
or  sheet,  may  be  gradually  raised  from  the  electrolyte  while 
being  washed  with  cold  water,  the  current  not  being  broken.  This 
procedure  is  in  some  cases  easier  than  the  usual  process  of 
siphoning.  Washing  with  alcohol  and  ether  is,  of  course,  carried 
out  as  usual  before  weighing.  Such  a  course  is  of  decided  ad- 
vantage when  the  sulphur  is  to  be  determined  as  well  as  the 
metal,  thereby  avoiding  a  large  dilution  of  the  sulphate  solution. 

There  is  no  reason  for  using  gauze  of  nickel-plated  platinum, 
a  nickel  gauze  serving  just  as  well.  The  plated  nickel  electrode 
was  used  in  this  work  because  it  was  at  hand  and  a  good 
nickel  coating  was  easily  made  from  an  ammoniacal  double  sul- 
phate solution  of  nickel  and  ammonium,  and  served  for  numerous 
determinations.  The  cheapness  of  a  nickel  cathode  is,  however, 
a  decided  advantage. 

While  it  has  been  shown  that  determinations  of  zinc  in  a  pure 
blende  can  be  made  without  filtering,  using  a  vertical  gauze 


11 

cathode  and  correcting  slightly  for  iron,  yet  it  is  generally  better 
to  filter,  and  is  really  a  necessity  if  the  amount  of  insoluble  mat- 
ter is  large. 

Determination  of  Sulphur. — The  electrolytes  remaining  after 
the  deposition  of  the  metal  in  Nos.  37  and  38  were  concentrated, 
made  acid  with  hydrochloric  acid,  and  after  filtration  to  re- 
move silica  and  gangue,  barium  chloride  was  added  with  the 
usual  precautions,  giving: 

Number        Zince  blende        Barium  sulphate        Sulphur  found        Sulphur 
gram  gram  gram  per  cent. 

37  0.4454  1.0347  0.14206  31.89 

38  0.4178  0.9722  0.13348  31.94 
A  slight  trace  of  sulphate  was   found  in  the  potassium  hy- 
droxide used  in   fusion. 

Gravimetric  analysis  of  this  same  blende  showed  : 

Gangue   1.18%     (1.18)% 

Iron    Fe 0.41  o  .41 

Sulphur    S 31.72  31.79 

Zinc    Zn 64.96  65.10 

Calcium  carbonate. .  .CaCOs 1.55  (i-55) 

99.82         100.03 

Gangue  and  calcium  carbonate  were  determinted  in  only  one 
sample,  the  latter  being  calculated  from  calcium  oxide.  The 
ore  was  carefully  oxidized  by  dilute  nitric  acid;  this  was  re- 
placed by  hydrochloric  acid,  and  after  filtering  out  the  gangue 
and  later  the  ferric  hydroxide  precipitated  by  ammonium  hy- 
droxide, the  zinc  was  precipitated  as  sulphide.  This  zinc  sul- 
phide was  dissolved  with  dilute  nitric  acid  from  off  filter  into 
weighed  porcelain  crucible,  evaporated  to  dryness  on  water  bath 
and  ignited  with  blast  to  constant  weight  as  zinc  oxide. 

A  comparison  of  methods  shows : 

Zinc  (percentages)  Sulphur 

Electrolytic  Gravimetric  From  siphonate  Oxidized  by 

nitric  acid 

64.87  64.96  31.89  31.72 

64.93  65.10  31-94  3L79 

64.95 

Sulphur  from  the  siphonate  is  high,  doubtless  from  the  trace  of 
sulphur  in  potassium  hydroxide. 

It  will,  therefore,  be  seen  that  it  is  possible  to  determine  the 
metal  from  a  pure  zinc  blende  and  that  in  the  siphonate  all  the 
sulphur  is  completely  oxidized  and  may  readily  be  determined, 


12 

thus  giving  both  chief  constituents  more  easily  and  quickly  than  is 
possible  by  other  methods.  The  ideal  process  of  determining  the 
sulphate  ion  electrolytically  from  this  solution  is  yet  to  be  found. 
The  easy  and  accurate  method  of  determining  sulphate  by 
titrating  the  acid  liberated  in  the  electrolysis  of  the  salt  is  hardly 
applicable  here  in  the  large  excess  of  potassium  hydroxide  re- 
quired. 

After  weighing  out  the  ore  an  hour  should  suffice  for  the 
zinc  determination  in  a  pure  blende,  but  by  carrying  on  a  number 
of  analyses  at  the  same  time  it  should  be  possible  to  considerably 
reduce  the  average  time. 

APPLICATION   OF  THE  METHOD   TO   OTHER  ZINC 

ORES. 

The  method  having  proved  successful  for  a  pure  blende,  its 
general  applicability  to  zinc  ores  was  now  investigated. 

The  nickel-plated  platinum  gauze  served  as  cathode  and  the 
same  vertical  spiral  of  platinum  as  anode.  The  fusion  was  taken 
up  in  water  and  filtered  through  a  Gooch  crucible;  before  elec- 
trolyzing,  this  solution  was  heated. 

II.  An  ore  from  Phoenixville  containing  sphalerite  and  wurt- 
zite;  less  pure  than  I. 

Number    Zinc  blende    Time  heated  Current  Zinc  found    Zinc 

gram  minutes       Volts  Amperes  Time        gram          per 

cent. 

39  0.5553  35  4-5        5          40          0.3230        58.17 

40  0.5574  45          0.3249        58.li 


41  0.7690  4  40          0.4606  59-89 

42  0.4958  45  0.2978  60.06 

43  0.5596  30  40          0.3357  59-99 

44  0.4611  0.2776  60.20 

45  0.5949  35  0.3558  59-8i 

Average — 5  determinations  59-99 

In  Nos.  39  and  40  some  zinc  remained  in  the  residue  on  the 
Gooch  crucible,  being  held  up  by  ferric  hydroxide  and  other  in- 
soluble substances.  In  Nos.  41-45,  inclusive,  the  residue  was  dis- 
solved from  off  the  filter  in  hydrochloric  acid  and  reprecipitated 
with  excess  of  potassium  hydroxide,  the  second  filtrate  being 
added  to  the  first.  It  is  well  to  evaporate  this  hydrochloric  acid 
solution  nearly  to  dryness  to  drive  off  the  excess  of  acid  before 
reprecipitating  with  alkali.  In  these  last  five  determinations, 


13 

which  were  successive,  there  was  no  zinc  left  in  final  residue  on 
the  Gooch,  nor  in  the  solution  after  electrolysis ;  no  iron  in  zinc 
deposit.  A  partial  analysis  made  gravimetrically  showed  3.65% 
gangue  and  2.93%  iron. 

It  is  evident  then  that  if  there  is  even  so  much  as  half  of 
one  per  cent,  of  iron  that  it  is  necessary  to  dissolve  and  re- 
precipitate  the  ferric  hydroxide  to  free  it  from  zinc. 

From  the  beginning  of  the  work  it  was  thought  that  the  separa- 
tion from  the  lead  might  be  difficult,  because  of  the  formation 
of  potassium  plumbite,  soluble  in  excess  of  alkali  just  as  is  the 
potassium  zincate.  After  four  or  five  non-concordant  determ- 
inations were  obtained  on  a  mineral  containing  galenite,  the  ore 
was  completely  analyzed  to  find  out  just  what  were  the  condi- 
tions. 

III.  An  ore  from  Joplin  containing  sphalerite,  wurtzite,  gal- 
enite and  calcite.  On  analysis  this  gave : 

Gangue   1.11% 

Sulphur    S    21.05 

(precipitated  as  BaSO*) 
Sulphur  S      3.22 

(from  PbSO4) 

Iron    Fe 

Manganese    Mn 

Zinc    Zn 

Lead    Pb 

Calcium  carbonate  CaCO  3 

(from  CaO) 

100.12 

From  the  many  electrolytic  determinations  which  were  at- 
tempted the  following  may  be  given : 

Number       Zinc  ore       Time  heated  Current  Zinc  found    Zinc 

gram  minutes       Volts  Amperes  Time        gram  per 

cent. 
46  0.3445  40  S  5          40  0.1503        43.62 


47  0.4269 

48  0.5409 

49  0.4521 

50  0.5882 

51  0.5753 


0.1838  43.05 

0.2416  44.66 

0.2157  47.71 

5-5  35          0.2727  46.36 

5  0.2592  45.05 


In  Nos.  46  and  47  the  fusion  was  taken  up  in  water  and  filtered 
directly  from  the  crucible.  The  filtrate  at  first  clear,  clouded  on 
dilution  with  wash  water,  the  precipitate  containing  lead  probably 
as  the  hydroxide.  The  residue  on  the  filter  was  not  dissolved  and 


14 

reprecipitated  and  the  zinc  held  back  by  the  iron  was  compensated 
for  by  the  lead  deposited,  the  correct  result  being  a  mere  coinci- 
dence due  to  the  balancing  of  errors. 

In  No.  48  the  fusion  was  treated  with  water,  poured  into  a 
beaker,  diluted  and  boiled  before  filtering.  This  was  done  to 
save  the  additional  filtration  to  remove  the  precipitate  formed  on 
dilution,  but  there  was  more  lead  in  the  deposit  than  in  the  two 
preceding  cases.  An  increase  in  the  original  weight  of  the 
cathode  after  dissolving  the  zinc  in  the  dilute  sulphuric  acid 
always  indicated  the  presence  of  lead  before  a  test  had  been  made 
for  it. 

An  attempt  was  made  in  No.  49  to  completely  precipitate  the 
lead  as  lead  dioxide  by  adding  potassium  permanganate,  precipi- 
tating the  excess  of  manganate  as  the  dioxide  with  alcohol,  dis- 
solving and  reprecipitating,  but  without  success. 

In  No.  50  a  similar  attempt  at  oxidation  was  made  with  hy- 
drogen peroxide  and  in  No.  51  by  fusion  with  addition  of  sodium 
peroxide  with  the  same  results.  An  effort  was  made  to  determ- 
ine the  zinc  by  noting  increase  in  cathode  weight  after  treat- 
ment with  sulphuric  acid  as  lead,  and  deducting  this  from  the 
total  weight  of  the  deposit,  but  the  results  were  not  reliable. 

It  is  noteworthy  that  the  amount  of  lead  deposited  is  always 
less  than  5%,  though  the  lead  present  in  the  ore  is  nearly  21%. 
This  seems  to  be  due  chiefly  to  the  formation  of  the  white  pre- 
cipitate mentioned  above. 

There  seemed  to  be  no  lead  in  the  final  zinc  deposit  when  this 
procedure  was  adopted :  The  first  zinc  deposit  containing  some 
lead  was  dissolved  in  dilute  sulphuric  acid  without  drying  and 
weighing.  After  adding  alcohol  to  this  solution  it  was  filtered; 
to  the  filtrate  an  excess  of  potassium  hydroxide  was  added  and 
the  solution  warmed  and  electrolyzed.  Time  for  fully  working 
out  this  process  was  lacking.  Further  work  will  be  done  with 
these  lead-zinc  ores,  and  it  is  believed  that  the  difficulty  is  one 

that  may  be  overcome. 

* 

IV.  More  satisfactory  results  were  obtained  on  the  appli- 
cation of  the  method  to  a  sample  of  franklinite  from  the  New 
Jersey  Zinc  Co. 


15 


Number 


52 
53 

54 

It 

57 


Zinc  ore 
gram 

0.5173 
0.5260 

0.7822 
0.5079 
0.5826 
0.5218 


Time  heated  Current  Zinc  found    Zinc 

minutes       Volts  Amperes  Time        gram          per 

cent. 

40  0.0804        15.54 

0.0810         15.40 


12 


40 


35 
40 

20 


0.1233 
0.0832 
0.0938 
0.0853 


Average — 4  determinations  16.15 


On  account  of  much  manganese  and  iron  hydroxides,  the  resi- 
due on  the  Gooch  crucible  was  dissolved  in  hydrochloric  acid  and 
reprecipitated  as  in  II.  The  addition  of  alcohol  for  the  purpose 
of  precipitating  the  hydrated  dioxide  from  the  maganate  is  neces- 
sary before  both  filtrations. 

In  Nos.  52  and  53  there  was  trouble  in  the  zinc  deposition,  the 
metal  being  deposited  only  around  the  bottom  of  the  cathode  and 
probably  not  entirely  adherent.  The  hydrochloric  acid  solution 
had  not  been  evaporated  to  remove  excess  of  acid  before  addition 
of  alkali  and  the  potassium  salts  thus  formed  so  greatly  in- 
creased the  conductivity  as  to  render  it  difficult  to  obtain  a  high 
enough  voltage.  On  evaporating  off  the  excess  of  acid  and  re- 
plating  the  cathode  with  nickel,  further  trouble  was  avoided, 
though  a  higher  amperage  than  that  used  with  other  ores  was 
required. 

There  was  no  zinc  in  the  siphonate,  nor  iron  in  zinc  deposit. 
No  trace  of  maganese  was  found  on  either  anode  or  cathode. 
As  far  as  could  be  told  with  an  excess  of  manganese  and  iron, 
there  was  no  zinc  in  the  residue  on  the  filter. 

V.  Two  determinations  on  a  sample  of  willemite  from  the 
New  Jersey  Zinc  Co.  show  the  method  applicable  to  that  mineral. 

Number       Zinc  ore       Time  heated  Current  Zinc  found    Zinc 

gram  minutes       Volts  Amperes  Time        gram          per 

cent. 

58  0.5598       30      4.5    5     40    0.2808    50.16 

59  0.6063  35     0.3018   49.78 

The  residue  on  the  filter  was  dissolved  and  reprecipitated  as 
usual.  There  was  no  zinc  in  final  residue  nor  in  siphonate ;  no 
iron  in  zinc  deposit.  The  samples  from  the  New  Jersey  Zinc  Co. 
were  received  through  the  courtesy  of  Mr.  George  C.  Stone,  to 
whom  thanks  are  due. 


16 

The  volume  of  the  solutions  electrolyzed  varied  from  100  c.  c. 
to  250  c.  c.  This  is  not  relatively  of  great  importance ;  in  general, 
heat  and  concentration  favor  a  rapid  deposition.  The  fusions 
were  done  in  nickel  crucibles,  which  are  but  slightly  attacked  by 
the  fused  alkali,  as  many  as  fifty  determinations  being  made  in 
the  same  crucible.  And  this  in  spite  of  the  fact  that  the  fusions 
were  usually  heated  longer  than  necessary,  being  sometimes  al- 
lowed to  run  over  a  lecture  hour. 

To  eliminate  platinum  entirely  from  the  apparatus  required, 
an  electrolysis  of  a  standard  zinc  solution  was  made,  using  a  spiral 
of  nickel  wire  as  anode  and  a  cathode  of  nickel  gauze.  The  de- 
termination was  satisfactory  and  the  anode  being  weighed  both 
before  and  after  the  deposition  showed  no  loss  in  weight. 


CONCLUSION. 

This  work  has  brought  out  several  points  of  interest: 

1.  That  the  metal  in  a  pure  zinc  blende  may  be  determined 
easily,  rapidly  and  accurately  in  the  electrolytic  way  after  fusion 
with  a  caustic  alkali,  and  that  in  the  siphonate,  or  solution  left 
after  electrolysis,  the  sulphur  is  completely  oxidized  and  ready  for 
estimation  in  the  usual  manner  with  barium  chloride.  The  method 
is   shorter  than   others,   whether  electrolytic  or  not,   when  the 
metal  alone  is  considered  and  possesses  the  additional  advan- 
tage of  the  sulphur  being  available  for  determination. 

2.  The  process  is  applicable  to  zinc  ores  generally  with  the 
possible  exception  of  those  containing  lead.     There  is  practically 
no  opportunity  for  the  entering  of  the  personal  element,  for  the 
current  causes  the  deposit  and  the  balance  fixes  its  amount.   Con- 
centration, temperature  and  exactly  precise  conditions  have  little 
influence  and  need  not  be  constantly  watched.    The  method  works 
with  equal  ease  for  rich  or  poor  ores. 

3.  By  the  use  of  nickel  electrodes,  not  only  more  satisfactory 
deposits  are  obtained  but  the  cost  of  apparatus  is  reduced  to  a 
minimum.     This  removes  an  objection  frequently  urged  against 
electrolytic  methods.     The  cylindrical  cathode  of  nickel  gauze 
dipping  vertically  into  the  solution  in  a  beaker,  is  preferable  to 
the  dish  or  vertical  sheet  of  metal. 

The  possibility  of  the  use  of  electrolytic  methods  in  analyses 
of  mineral  products  has  been  emphasized.  Further  work  should 
be  undertaken  along  this  line. 


RETURN 


CIRCULATION  DEPARTMENT 

198  Main  Stacks 


LOAN  PERIOD  1 
HOME  USE 

2 

3 

4 

5 

6 

ALL  BOOKS  MAY  BE  RECALLED  AFTER  7  DAYS. 

Renewls  and  Recharges  may  be  made  4  days  prior  to  the  due  date. 

Books  may  be  Renewed  by  calling  642-3405. 

DUE  AS  STAMPED  BELOW 


• 
I 


FORM  NO.  DD6 


UNIVERSITY  OF  CALIFORNIA,  BERKELEY 
BERKELEY,  CA  94720-6000 


